Browsing Environmental Sustainability Research Centre by Subjects
Now showing items 1-3 of 3
A chronology of alluvial fan response to Late Quaternary sea level and climate change, CreteTo better understand how fluvial systems respond to late Quaternary climatic forcing OSL and U-series dating was applied to stratigraphically significant sedimentary units within a small (<6.5 km) alluvial fan system (the Sphakia fan) in southwest Crete. The resultant chronology (comprising 32 OSL and U-series ages) makes Sphakia fan one of the best dated systems in the Mediterranean and suggests that Cretan fans responded to climate in two ways. First, during the transitions between Marine Isotope Stage (MIS) 5a/4 and MIS 2/1 Sphakia fan was characterised by significant entrenchment and distal shift in the zone of deposition. It is proposed that the phases of entrenchment were driven by sea level induced base level fall during MIS 5a/4 and landscape stabilisation during the onset of the current interglacial (MIS 2/1). Second, with the exception of these two entrenchment episodes fan alluviation occurred across the entire last interglacial/glacial cycle in all climatic settings i.e. interglacials, interstadials and stadials. It is likely that the topographic setting of the catchment supplying sediment to Sphakia fan maintained high sediment transfer rates during most climatic settings enabling fan aggradation to occur except during major climatic driven transitions i.e. major sea level fall and postglacial vegetation development.
Clarifying stages of alluvial fan evolution along the SfakianAnalysis of fan sediments and post-incisive soils was combined with luminescence dating to re-assess Nemec and Postma's [Nemec, W., Postma, G., 1993. Quaternary alluvial fans in southwestern Crete: sedimentation processes and geomorphic evolution.In: Marzo, M., Puigdefábregas, C. (Eds.), Alluvial Sedimentation. Special Publication of the International Association ofSedimentologists, vol. 17, pp. 235–276] model of fan evolution on the Sfakian piedmont, southern Crete. Field mapping supportsthe assertion that sedimentation occurred in three developmental stages. Stage 1 sediments comprise angular debris flows formingsmall cone-like deposits; stage 2 fluvial gravels form large, relatively steep streamflow-dominated telescopic fans; and stage 3sediments consist of coarse sieve-type alluvium, localised mudflows and hyperconcentrated flow deposits. Irrespective of gradient, fan surfaces are capped by post-incisive soils that form a chronosequence comprising remnant chromic luvisols. The most developed profiles, highest redness rating, and greatest concentrations of Fed and magnetic minerals are associated with soils formed on stage 1 surfaces. The stage 2 and 3 soils record progressively lower redness rating, Fed, and magnetic values, indicating that the stage 1 soils and fan surfaces formed first, followed by stage 2 and 3 soils and fan surfaces. Nanofossil data strongly suggest that stage 1 sedimentation commenced no earlier than the Early Pleistocene. Optically stimulated luminescence (OSL) results suggest that sedimentation responsible for stage 2 surfaces occurred between Marine Isotope Stage (MIS) 6 and MIS 2, while archaeological data indicate that stage 3 sedimentation is of Holocene age. The re-investigation of fan sediments and morphology corroborates the sedimentary and morphological elements of Nemec and Postma's model. The soil data support the model's assumptions that sedimentation was broadly synchronous across the piedmont development and controls fan incision. Local uplift resulted in variable rates of incision that culminated in differential fan segmentation across the piedmont. and the locus of deposition progressively shifted away from the range-front zone. OSL dating suggests that previous age estimates assigned to fan stages 1 and 2 are too old. Climate appears to exert a fundamental control over fan development, with sedimentation occurring during cold stages and cold stage-interglacial transitions. Tectonic activity provides the relief required for fan development and controls fan incision. Local uplift resulted in variable rates of incision that culminated in differential fan uplift
Quantitative mapping of alluvial fan evolution using ground-based reflectance spectroscopyThe ability of field-based reflectance spectroscopy to resolve the relative proportions of Fe-oxides and clays in soils was used to map the composition, relative age and distribution of segments within late Quaternary fan systems in Sfakia, southwest Crete. The spectrometric results demonstrate that luvisols that have formed on the surfaces of fan segments are characterized by distinctive Fe-oxides (types) and clay minerals (species). Furthermore, Fe-oxide and clay concentrations display a clear and consistent trend whereby for each study fan luvisols formed on increasingly proximal fan segments are characterized by a progressive build-up of spectrally distinct secondary iron oxides and clay minerals, which suggests that proximal segments formed first. The relative ages and hence order of formation of segments suggested by the spectral data are strongly supported by an optically stimulated luminescence (OSL)-based geochronology which provides a tentative maximum age of 144 ka for the oldest (stage 1) surface and 11.2 ka for the youngest (stage 2C) surface. Moreover, the chronometric data indicate that time intervals of the order 20 to 25,000 years are necessary to generate sufficient differences in pedogenic Fe-oxides and clay concentrations to enable differentiation of fan segments by field spectroscopy. âº Field-based reflectance spectroscopy was used to map alluvial fan segments. âº Strong relationships between spectral profiles and soil composition were identified. âº Fan segments were differentiated using time interval between depositional events. âº Age differences of 25,000 years are required to differentiate fan segments.